patch 8.0.1236: Mac features are confusing

Problem:    Mac features are confusing.
Solution:   Make feature names more consistent, add "osxdarwin". Rename
            feature flags, cleanup Mac code. (Kazunobu Kuriyama, closes #2178)
diff --git a/src/ui.c b/src/ui.c
index f175e1a..d7daf94 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -1748,7 +1748,7 @@
     void
 fill_input_buf(int exit_on_error UNUSED)
 {
-#if defined(UNIX) || defined(VMS) || defined(MACOS_X_UNIX)
+#if defined(UNIX) || defined(VMS) || defined(MACOS_X)
     int		len;
     int		try;
     static int	did_read_something = FALSE;
@@ -1772,7 +1772,7 @@
 	return;
     }
 #endif
-#if defined(UNIX) || defined(VMS) || defined(MACOS_X_UNIX)
+#if defined(UNIX) || defined(VMS) || defined(MACOS_X)
     if (vim_is_input_buf_full())
 	return;
     /*